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When do the four seasons officially begin? (FAQ - Time)

There are no 'official' definitions of the duration of spring, summer, autumn and winter for civil purposes. Many bodies, for example meteorologists, adopt a convention for the purpose of presenting statistics by grouping the twelve months of the year into four three-month seasons, for example March, April and May being taken as Spring.

The astronomical events closely related to the four seasons are the equinoxes and the solstices, and these have been used to define the seasons 'astronomically'. The equinoxes occur in March and September when the Sun is 'edgewise' to the Earth's axis of rotation so that (neglecting the effect of atmospheric refraction) everywhere on Earth has twelve hours of daylight and twelve hours of darkness. The solstices occur in June and December when the Earth's axis is at its extreme tilt towards and away from the Sun so at mid-day it appears at its highest in one hemisphere and at its lowest in the other.

These four events repeat every 'tropical' year (365 days, 5 hours, 49 minutes), so they become later by about six hours, or (if there has been an intervening leap day) earlier by about 18 hours, from one year to the next. They are not equally spaced in the year, because the Earth's orbit around the Sun is elliptical, not circular. Their timings can be obtained, for example, from Whitaker's Almanack or a range of websites. For 2013 they were:

Date

Northern hemisphere

Southern hemisphere

March 20 at 11:02 UTC

Vernal (Spring) equinox

Autumnal equinox

June 21 at 05:04 UTC

Summer solstice

Winter solstice

September 22 at 20:44 UTC

Autumnal equinox

Vernal (Spring) equinox

December 21 at 17:11 UTC

Winter solstice

Summer solstice

There are many sources (for example, diaries) which use these dates as if they were the boundaries of the seasons, for example 'June 21st, Summer begins', but this leads to confusion when, in many European countries, June 24th is celebrated as midsummer day.